1. Field Of The Invention
This invention relates to the process for the preparation of carboxylic acid chlorides of the benzene series. More especially, this invention is directed to an improvement in the preparation of such carboxylic acid chlorides prepared by the use of a molybdenum catalyst, the improvement residing in introducing a complexing agent into the reaction mixture prior to the time that the carboxylic acid chloride is distillatively removed.
2. Discussion Of The Prior Art
In parent application Ser. No. 737,368 there is described a method for the preparation of carboxylic acid chlorides of the benzene series by reaction with the corresponding carboxylic acid ester with an aromatic compound containing at least one nuclearly bound trichlormethyl group in the presence of a molybdenum compound as catalyst. In this reaction it is desirable to remove by distillation from the reaction mixture the alkyl chloride that forms in the reaction and after the end of the reaction to remove the carboxylic acid chloride by distillation, preferably in vacuo.
The acid chlorides which are distilled off contain more or less large quantities of molybdenum. The higher the boiling point of the acid chloride is the higher is the molybdenum content of the distillate and of the desired acid chloride. Thus, it is possible to obtain p-toluylic acid chloride free of molybdenum by distillation while terephthalic acid dichloride obtained by the same process is distilled off in a distillate containing 45 ppm of molybdenum. Similarly, when one attempts to prepare trimellitic acid trichloride by this route employing the molybdenum catalyst the resultant distillate contains 80 ppm of molybdenum. Even when the distillation is performed quite carefully in a column, the molybdenum content is quite high, i.e., is only slightly reduced from those runs performed without the use of a column.
These small quantities of molybdenum in the distillate are undesirable for they necessitate further processing of the carboxylic acid chlorides. In addition, a molybdenum content of more than 40 ppm results in a more or less intense reddish-brown discolored product. Such a product requires extensive further treatment.
It is an object of this invention, therefore, to provide an improved process for the preparation of substituted or unsubstituted carboxylic acid chlorides of the benzene series by reaction of the corresponding carboxylic acid ester with an aromatic compound containing at least one nuclearly bound trichlormethyl group in the presence of a molybdenum catalyst wherein the distillate containing the desired acid chloride product is virtually free of molybdenum. It is another object of this invention to provide such a distilled product wherein the molybdenum content of the distillate is well below 5 ppm.